This invention generally relates to a scrubber for blast furnace gas, by means of which a purification and loss of gas pressure can be accomplished prior to introducing such a gas into an industrial utilization system and which also permits pressure adjustments at the throat of the blast furnace during operation of the latter.
As the pressure at the blast furnace throat increases, there is an increasing need for equipment capable of adjusting the furnace pressure with smaller and smaller percentage differentials and dissipating the gas energy, prior to reusing the gas, with minimal noise generation and minimal wear of the adjustment members.
Purification systems are known which comprise a plurality of parallel throttle valves which adjust and reduce the pressure of the gas previously separated from the scrubbing or washing water, and purification systems are also known which comprise adjustment members within the system for adjusting the gas pressure at a single stage in the presence of water.
The adjustment characteristics of the former system, using throttle valves, proved to be unsatisfactory, particularly at high pressures, because the adjustment characteristics of the system operating at high pressures were extremely sensitive to the displacements of the adjustment members.
Other known purification systems have pressure stage or pressure drop characteristics of their own which are functions of the stroke of the adjustment members, and these systems have proven superior to the aforesaid devices. By controlling the full pressure imparted to the gases, these systems provide acceptable adjustment by controlling the cross-sectional areas of least change. However, these acceptable adjustments become substantially impossible as a result of high pressures that may occur at the blast furnace throat, and particularly in case of wear of this structure.
Additionally, as to wear strength and noise generation, known systems behave quite unsatisfactorily particularly at high pressures, when the gas speed astride the throttling members can attain hypercritical rates or rates close to critical, with the expectable consequences of vibration, noise, and effects upon adjustment accuracy.
The present invention is directed to provide a scrubber for blast furnace gas suitable to improve the adjustment characteristics of the pressure stage and capable of assuring less wear of the adjustment members and a reduced noise generation, since the actual gas speed astride the throttling members is definitely lowered at the critical conditions.
Generally, according to the invention, a scrubber for blast furnace gas comprises a first gas purification unit for providing a first purification in the presence of water, and a second gas purification unit, in series with the former, for providing a second purification in the presence of water and a pressure adjustment, which second purification unit comprises at least one multi-stage venturi including a first stage having a constant annular passage followed by a diffuser and at least a second adjustable stage subsequent to the first stage and defining an annular passage movable and gradually adjustable along a diffuser.
Particularly, the narrow gas passage section or annular groove in the second stage has a larger area than that of the groove in the preceding stage of the multi-stage venturi, and the purification stages are dimensioned, so that the first stage having a constant groove is capable of providing at the minimal furnace pressure, a sufficient pressure drop to obtain the required degree of purification, and so contrived that, as the movable adjustment member starts to move forward, the stage immediately following will create an annular groove or passage which becomes narrower at a determined rate. At a predetermined point further forward movement of the adjustment member does not involve any further narrowing of the groove, but provides a subsequent purification stage behaving as the first purification stage.
Therefore, according to the invention, at the minimal pressure of the furnace the movable adjusting elements do not interfere with the gas flow and the required pressure drop is provided by the first constant groove or passage of the multi-stage venturi. As the pressure, and accordingly the gas density increases, the constant groove provides a loss of load increasing with said pressure. The difference between the desired loss and the loss provided by the first stage of venturi, is obtained by gradually registering or adjusting the minimum passage section of the next adjustable stage or stages, each of which in its adjustment range should create a reduced loss of load than the total increment of loss of load, as a portion of the loss increment has been provided by the preceding stage or stages. This would result in an improvement in the sensitivity and adjustment characteristics of the apparatus.
According to a particular embodiment, the multi-stage venturi comprises a fixed or stationary portion and a movable portion slidably disposed therein. The stationary portion includes a divergent housing formed of a series of cylindrical elements interconnected by frusto-conical elements of varying cross-sectional area, the cross-sectional area increasing in the same direction as the gas flow through the venturi. The movable portion has a number of annular lugs or projections of increasing diameter disposed at axially-spaced positions, which cooperate with the corresponding sections of the stationary portion to define annular grooves or narrow sections for the passage of the gas.